Aliphatic polyester composition

ABSTRACT

A polymer composition comprises an aliphatic polyester and at least one compound of the general formula (I): R 1   a  (A)R 2   b , where A is a polyfunctional hydrocarbon group; R1 is an electron-donating group such as hydroxy, amino, thiol, or a derivative thereof; R2 is an electron-withdrawing group such as carboxylic acid, carboxylic amide and quaternary onium; and a and b are at least 1.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is the national phase entry of International patentapplication PCT/GB95/01375, filed Jun. 13, 1995, and claims priorityfrom Great Britain patent application no. 9411792.6, filed on Jun. 13,1994.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

THIS INVENTION relates to a polymer composition and in particular to acomposition comprising an aliphatic polyester and a stabiliser againstthermal decomposition.

2. Description of Related Art

Examples of such polyesters have become available commercially as theresult of the development of microbiological processes for making them.The earliest example, PHB, was difficult to melt-process owing to lowstability at its melting temperature. Corresponding copolyesters melt ata lower temperature. In addition, synthetic aliphatic polyesters of thehead-to-head tail-to-tail type such as polybutylenesuccinate and of thehead-to-tail type such as polylactic acid, polycaprolactone andsynthetic polyhydroxyalkanoates, have recently become available. Thereis room for improvement in stability, such that greater freedom inchoosing processing conditions and/or a wider range of end-uses couldbecome available. This has recently arisen using the stereospecificpolyesters produced microbiologically.

BRIEF SUMMARY OF THE INVENTION

It has now been found that compositions comprising such polyesters(which term includes homopolyesters, copolyesters and mixtures thereof)and one or more polyfunctional compounds of a defined class showsubstantial advantages.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 to 5 of the accompanying drawings show the DSC results obtainedin respectively runs 1 to 5 of the Example.

DETAILED DESCRIPTION OF THE INVENTION According to the invention apolymer composition comprises an aliphatic polyester and at least onecompound of general formula I:

    R.sup.1.sub.a (A) R.sup.2.sub.b                            I

where A is a polyfunctional hydrocarbon group;

R¹ is an electron-donating group;

R² is an electron-withdrawing group; and

a and b are at least 1.

Hydrocarbon group A is preferably aliphatic, preferably containing ahydrocarbon chain of up to 8 carbon atoms with up to 6 carbon atoms inany side chain(s).

Groups R¹ are suitably selected from hydroxy, amino, thiol andderivatives thereof. Such an amino group preferably is primary or issecondary or tertiary carrying one or two lower alkyl (C₁₋₆) groups. Itmay be present in part in protonated form, especially in presence ofweak acid such as carboxylic, for example in a zwitterion. Suitablederivatives are esters or amides, as mentioned further below. In suchester or amide the linkage to A is through O or N, respectively. GroupsR¹ in any one such compound may be the same or different.

Groups R² preferably include at least one carboxylic acid and/orcarboxylic amide and/or quaternary onium group, for example ammoniumquaternised with lower alkyl, especially methyl. In such amide thecarbon of the carboxylic group is linked to A directly. Group R² may bethe same or different in any one such compound.

The numbers a and b preferably total 2 to 4.

The compound is preferably an amino carboxylic acid. Any one or more ofthe "essential" amino acids may be used, or corresponding acids ofdifferent chirality or in racemic form. Alternatively synthetic aminoacids may be used. Preferably there is at least one group R¹ additionalto the CO₂ H and β-amino group. Thus it is preferably for exampleasparagine, aspartic acid, arginine, cysteine, glutamic acid, glutamine,histidine, lysine, ornithine, serine, threonine, tryptophan or tyrosineor a mixture of two or more of these possibly with other amino acids.

When R₂ is a quaternary onium group a preferred compound is aquaternised alkanolamine, for example choline or a derivative thereof,especially a phosphatidyl choline.

If, instead of or in addition to the free compound, a derivative thereofis present, R¹ may be for example an ester or amide derived from a C₁₋₆aliphatic carboxylic or carbonic or carbamic or phosphoric (includingphosphatidyl) acid. Likewise R² may be an ester derived from a C₁₋₆alcohol or an amide derived from ammonia or a C₁₋₆ amine. A compoundoxidisable or reducible to the defined compound in accessible conditionscan be used, for example cystine in place of cysteine. It is possiblethat such a derivative is a precursor decomposable to the definedcompound during processing and/or ageing of the polyester. Likewise, athermal and/or oxidative reaction product of the compound, such as mayresult from processing and/or ageing, may be added instead of freecompound or derivative.

The compound may be a condensation polymer having R¹ _(a) (A) R² _(b) asrepeating unit, such as a synthetic or natural peptide or protein. Suchpeptide or protein may be the same as or different from proteinaceousmaterial present with polyester in a microbiological fermentationproduct or resulting from treatments to recover polyester therefrom. Ifthe same, then the polyester component is synthetic or ismicrobiological material from which protein has been extracted or inwhich an enhanced protein content is required.

Particular examples of suitable compounds include glutamine,phosphatidyl choline and a mixture of cysteine, lysine, proline,ornithine, and serine. The phosphatidyl group may be for example1,2-diacylglycerophosphate ester, where the acyl groups are derived fromC₁₂₋₂₀ aliphatic monocarboxylic acids, or a corresponding mono-acyl(lyso) group.

The invention includes:

(a) the composition as defined above, as freshly prepared;

(b) the composition after processing steps short of conversion to afinished article; and

(c) a finished article:

in which there is present the defined compound and/or whateverconversion product has been formed from it.

The proportion of the defined compound to polyester is sufficient toprovide the desired stabilising effect, which depends on therequirements of intended processing conditions and end-use. Typically0.05 to 5, especially 0.1 to 2.0 phr of the compound is used. ("phr"denotes "per hundred resin", that is, parts by weight of compound perhundred parts by weight of aliphatic polyester).

The polyester (hereinafter referred to as "PHA") is preferably capableof a relatively high level of crystallinity for example over 30%,especially 50-90%, in the absence of plasticiser. It suitably consistsof or includes units of formula II:

    --O--C.sub.m H.sub.n --CO--                                II

where m is in the range 1-13 and n is 2m or (additionally when m is atleast 2) 2m-2. (It will be appreciated that n cannot be 2m-2 when m=1).Typically C_(m) H_(n) contains 2-5 carbon atoms in the polymer chain andthe remainder (if any) in a side chain. In very suitable polyesters n is2m and especially there are units with m=3 and m=4 copolymerisedtogether and having respectively a C₁ and C₂ side chain on the carbonnext to oxygen in the chain. Particular polyesters contain apreponderance of m=3 units, especially with 70-90 mol % of such units,the balance being units in which m=4. The value of m in the compositionmay be arrived at by blending component PHAs having mol percentage ofunits differing in m value. The molecular weight M_(w) of the PHA is forexample over 50000, especially over 100000, up to eg 2×10⁶.

In PHAs having units with m=3 and m=4 there may be very small, typicallyfractional, percentages of units having higher values of m. PHAconsisting essentially of m=3 units is poly-3-(R)-hydroxybutyrate (PHB),and PHA consisting of m=3 and 4 units is polyhydroxybutyrateco-valerate(PHBV).

The PHA can be a product of fermentation, especially of amicrobiological process in which a microorganism lays down PHA duringnormal growth or is caused to do so by cultivation in starvation of oneor more nutrients necessary for cell multiplication. The microorganismmay be wild or mutated or may have the necessary genetic materialintroduced into it. Alternatively the necessary genetic material may beharboured by an eukariote, to effect the microbiological process.Microbiologically produced PHA is (R)-stereospecific.

Examples of suitable microbiological processes are the following:

for Formula II material with m=3 or m=partly 3, partly 4:

EP-A-69497 (Alcaligenes eutrophus)

for Formula II materials with m=3:

U.S. Pat. No. 4,101,533 (A. eutrophus H-16)

EP-A-144017 (A. latus).;

for Formula II material with m=7-13:

EP-A-0392687 (various Pseudomonas).

The PHA can be extracted from the fermentation product cells by means ofan organic solvent, or the cellular protein material may be decomposedleaving microscopic granules of PHA. For specialised end uses thecellular protein may be partly or wholly allowed to remain with the PHA,but preferably subjected to cell breakage.

Alternatively the PHA can be synthesised, for example as described byBloembergen et al. in Macromolecules 1989, 22 1656-1663 (PHB) and1663-1669 (PHBV).

If the polyester is wholly or partly synthetic, it may be for example apoly-2-hydroxyalkanoate such as polyglycollic acid or polylactic acid ora synthetic poly -propio- or -butyro- or -valero- or -capro- or-pivalo-lactone or a copolymer comprising repeating units correspondingto any of these.

Examples of other suitable synthetic polyesters are especiallypolyesters containing combinations of dicarboxylic acids and diols. Suchdicarboxylic acids may be for example selected from the group consistingof malonic, succinic, glutaric, adipic, pimelic, azelaic, sebacic,fumaric, 2,2-dimethylglutaric, suberic, 1,3-cyclopentane dicarboxylic,1,4-cyclohexane-dicarboxylic, 1,3-cyclohexane dicarboxylic, diglycolic,itaconic, maleic, 2,5-norbornane dicarboxylic and combinations thereof.Such diols may be for example selected from the group consisting ofethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol,2,2-dimethyl-1,3-propanediol, 1,3 butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 2,2-trimethyl-1,6-hexanediol,thiodiethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol,2,2,4,4-tetramethyl-1,3-cyclobutanediol, triethyleneglycol,tetraethyleneglycol, di-, tri-, tetra- -propyleneglycol and combinationsthereof. In the production of such polyesters there may be usedester-forming derivatives of the acids and/or diols, instead of or inaddition to free acid and/or free diol.

The PHA of the composition preferably is of low nitrogen content, thatis, less than 5000 ppm, especially less than 2000 ppm w/w as N. Such PHAmay be for example the product of:

(a) decomposing and separating cell debris from fermentation product; or

(b) dissolving a cruder grade in an organic liquid and separatinginsolubles; or

(c) extraction from fermentation product by means of such a liquid.

PHA made microbiologically and harvested by removal of cell debris isdescribed in EP-A-0145233 and more recent co-pending applications.

The polyester component of the composition may contain more than onepolyester, for example:

(a) polyesters having the same repeating units but differing inmolecular weight; part of the polyester component may be of too low amolecular weight to be usable alone as structural material;

(b) polyesters having different combinations of repeating units;

(c) polyesters of a different class--e.g. synthetic withmicrobiological--but mutually miscible;

(d) polyester of a different class but not mutually miscible.

(e) polyester, whether or not differing as (a) to (d), having adifferent history, for example a different manufacturer or extractionprocedure or different previous processing such as re-work or recycle orend-group modification.

The composition may contain components in common use in plasticsprocessing, for example:

(a) one or more plasticisers;

(b) inorganic filler, for example glass fibre, carbon fibre, platy orfoil particle, silica, clay, magnesium silicate;

(c) organic filler, for example, cellulose fibre or particulate, proteinfibre, synthetic polymer particle or fibre, wood flour;

(d) polymer other than polyester;

(e) pigment;

(f) nucleant, especially boron nitride, talc, ammonium chloride orDZB/Zn stearate, at preferably 0.2 to 2.0 phr;

(g) volatile solvent for the polyester and compound.

If plasticiser is present, the ratio thereof to polyester depends on theintended use of the composition. The range 2-40 phr includes most of thelikely uses. For making effectively rigid but not brittle articles therange 5-20 especially 6-12 phr is generally suitable. Any of theplasticisers known for these polyesters or found subsequent to thisinvention would be suitable. Examples are:

(a) high boiling esters of polybasic acids, such as phthalates,isophthalates, citrates, fumarates, glutarate, phosphates or phosphites.The esterified radicals may be for example C₁ -C₁₂ alkyl, aryl oraralkyl. Particular examples are dioctyl-, diheptyl- and diundecyl-phthalates and dialkylalkylene oxide glutarate (Plasthall 7050);

(b) high boiling esters and part-esters of polyhydric alcohols,especially glycols, polyglycols and glycerol. The acid-derived radicalsof the ester typically contains 2-10 carbon atoms. Examples aretriacetin, diacetin and glyceryl dibenzoate;

(c) aromatic sulphonamides such as paratoluene sulphonamide.

A particularly preferred plasticiser is a doubly esterifiedhydroxycarboxylic acid having at least 3 ester groups in its molecule."Doubly esterified" means that at least some of the hydroxy groups ofthe hydroxycarboxylic acid are esterified with a carboxylic acid and atleast some of the carboxy groups thereof are esterified with an alcoholor phenol. Preferably at least the hydroxycarboxylic acid from which theester is derived is aliphatic or cycloaliphatic. Its backbone structure(that is, apart from carboxy groups) preferably contains 2-6 carbonatoms. It contains preferably 2-4 carboxy groups and 1-3 hydroxy groups;and preferably the number of carboxy groups exceeds the number ofhydroxy groups. An example of such a plasticiser is acetyl-tri-n-butylcitrate, available as Estaflex (Trade mark of AKZO Limited).

According to a further aspect of the invention a method of making thecomposition comprises blending the polyester with the compound.

This may be effected by for example:

(i) mixing the compound with the polyester in particulate form, forexample in particles smaller than 1000, especially smaller than 100,micrometers. Preferably the particles are in the size range 0.1 to 50micrometers. The particles are especially the product of solubilisingand removing non-polyester cell material from a microbiologicallyproduced biomass. The compound can be introduced at any convenientstage, including stages before the particles are isolated from aqueousmedium;

(ii) melting a mixture of polyester with compound. The mixture ispreferably as made by method (i). Other components, for example asabove, especially those that assist melting, may be present;

(iii) bringing the polyester and compound together in a volatile solventfor the polyester. The solvent may have been introduced as a means ofextracting the polyester from a microbiologically produced latex. It canthen be removed or not, according to the requirements of furtherprocessing.

Suitable solvents include cyclic carbonate esters and halogenatedhydrocarbons such as dichloromethane, chloroform and 1,2-dichloroethane.

A particular process comprises:

(i) forming a biomass of cells containing PHA granules and non-PHA cellmaterial by fermentation;

(ii) treating the cells to solubilise non-PHA cell material;

(iii) separating the PHA granules from the liquid phase;

and is characterised by introducing at least one compound ashereinbefore defined.

The process may include treating the granules with a peroxide whereby tosolubilise non-PHA material additional to what has been or will besolubilised in step (ii), and separating the granules from the resultingliquid phase. The compound may be introduced before the polyester hasbeen isolated from the latex produced by the fermentation. If thecompound is a condensation polymer, it may be a stabilising quantity ofnitrogenous material separated from the PHA granules.

In a yet further aspect the invention provides a process of making ashaped article by confining, eg in a mould or on a surface or through adie, a composition as defined above., Particular methods includeinjection moulding, compression moulding, extrusion of fibres or film,extrusion of profile, gas-current spinning, tack spinning, coating on tosubstrate, any of these being carried out, as appropriate, using thecomposition in the form of melt, particulate or solution in volatilesolvent. Examples of shaped articles made by such methods include filmsespecially for packaging, coated products (such as paper, paperboard andnon-woven fabrics), fibres, non-woven fabrics, extruded nets, personalhygiene products, bottles and drinking vessels, agricultural andhorticultural films and vessels, slow-release devices and ostomy bags.

Alternatively the composition with suitable additives can be used as anadhesive.

Test Procedures

Sample Preparation

(a) Mixing With Additive

Finely divided polyester (0.5 g) was mixed with compound, compressedinto blocks at 180° C. for 5 min, then cooled and held at 60° C. for 10min to allow crystallisation to occur.

(b) DSC Specimen

A compressed block was dissolved in chloroform at reflux temperature. Afew drops of the solution were cast into an aluminium pan of the DSCinstrument (see below) to give a specimen mass 5.6±0.1 mg.

DSC Examination

This technique measures the energy flow towards or from a specimen incomparison with an inert reference as a function of time andtemperature. A DSC instrument typically includes two isolated insulatedholders on each of which a pan containing respectively the test specimenor the reference is mounted. The heat flow is measured and compensatedfor, to maintain the temperature with respect of the reference.

A Perkin Elmer DSC-1B instrument was used, in dynamic mode using a 8° C.min⁻¹ temperature ramp, from 350K (77° C.) to ca. 550K (307° C.). A flowof oxygen was maintained over the specimens at 4.0 dm³ h⁻¹. The startingtemperature was held for about 2 min to allow for thermal equilibrium,before beginning the ramp.

Presentation of Results

The DSC instrument was linked to a Venture RE 511.20 chart recorder,which produced a graph of detector response (ordinate: arbitrary units,positive represents endotherm) against temperature (abscissa: K,negative represents exotherm attributed to oxidation).

FIGS. 1 to 5 of the accompanying drawings show the DSC results obtainedin respectively runs 1 to 5 of the Example.

EXAMPLE

Polyester compositions were prepared from a poly-3-hydroxybutyrate (PHB)of this specification:

Structure: PHB homopolymer (formula II, m=3);

molecular weight Mw 870000; Mn 228000;

origin: fermentation of glucose by Alcaligenes eutrophus;

extraction: enzymatic cell-debris removal;

purification: to under 100 ppm N.

The compositions and their behaviour were as follows:

1 PHB control without additives: there is a peak at 448K (175° C.) forthe endotherm of polymer melting.

Then follows, at up to about 482K (209° C.), a rather steep fall to 25units below the response level before the peak, showing that anexothermic process is taking place. An exotherm of such magnitude isnormally associated with oxidative degradation processes.

2 PHB with 5000 ppm w/w of the amino acid mixture (parts by weight):cysteine 1; lysine 7; ornithine 15; proline 13; serine 19.

3 PHB with 5000 ppm w/w of glutamine.

4 PHB with 5000 ppm w/w of phosphatidyl choline (XI-E from fresh eggyolk: P2772 in Sigma Chemicals Catalogue).

In 2-4 the melting peak endotherm is present as in 1. The onset of theexothermic oxidation peak is pushed significantly to highertemperatures, indicating that the additives stabilises the polyesteragainst exothermic decomposition.

5 PHB with 4000 ppm w/w of alanine: the trace is intermediate between 1and 2-4, showing that a moderate degree of stabilisation has takenplace.

We claim:
 1. A process for increasing the oxidation resistance of anR-stereospecific microbiologically produced polyester consisting of orincluding unite of the formula:

    --O--C.sub.m H.sub.n --CO--

where m is 3 to 4 and n is 2m, which comprises blending therewith atleast one compound of general formula:

    R.sup.1.sub.a (A) R.sup.2.sub.b

where A is a polyfunctional hydrocarbon group; R¹ is anelectron-donating group selected from hydroxy, amino and thiol andderivatives thereof; R² is an electron-withdrawing group selected fromcarboxylic acid, carboxylic ester, carboxylic amide and quaternaryammonium; and a and b are at least
 1. 2. A process according to claim 1,in which the content of the compound is 0.1 to 2.0 phr w/w.
 3. A processaccording to claims 1 or 2 in which the compound is selected fromaspartic acid, asparagine, glutamic acid and glutamine.
 4. A processaccording to claims 1 or 2 in which the compound is selected fromarginine, ornithine and lysine.
 5. A process composition according toclaim 4 in which the compound comprises lysine and ornithine inadmixture with at least one amino carboxylic acid selected fromcysteine, proline and serine.
 6. A process according to claim 1 or claim2 in which the compound is a phosphatidylcholine.
 7. A process accordingto claim 1 in which the polyester is capable of over 30% crystallinityin the absence of plasticiser.
 8. A process according to claim 7 inwhich the polyester m=3 units and m=4 units having respectively a C₁ andC₂ side chain on the carbon atom next to oxygen in the chain arecopolymerised together.
 9. A process according to claim 8 in which thepolyester contains 70-90 mol % of m=3 units, the balance being m=4units.
 10. A process according to any one of claims 7 to 9 in which thepolyester contains less than 2000 ppm w/w of nitrogen as N.
 11. Aprocess according to any one of claims 1, 2, 8, 9 or 10 which comprisesmixing the compound with the polyester in finely divided particle form.12. A process according to claim 11 in which the polyester particles arethe product of solubilizing and removing non-polyester material from amicrobiologically produced biomass.
 13. A polymer composition comprisingan R-stereospecific microbiologically produced polyester consisting ofor including units of the formula:

    --O--C.sub.m H.sub.n --CO--

where m is 3 to 4 and n is 2m and at least one compound of generalformula:

    R.sup.1.sub.a (A)R.sup.2.sub.b

where A is a polyfunctional aliphatic hydrocarbon group containing ahydrocarbon chain of up to 8 carbon atoms with up to 6 carbon atoms inany side chain(s); R¹ is an electron-donating group selected fromhydroxy, amino and thiol and derivatives thereof; R² is anelectron-withdrawing group selected from carboxylic acid, carboxylicester, carboxylic amide and quaternary ammonium; and a and b are atleast 1; subject to the conditions that either: a+b=2, R¹ is hydroxy oresterified hydroxy and R² is quaternary ammonium; or a+b=3 or 4, R¹ isselected from hydroxy, amino or thiol and R² is carboxylic acid, esteror amide.
 14. A composition according to claim 13 in which the contentof the compound is 0.1 to 2.0 phr w/w.
 15. A composition according toclaims 13 or 14 in which the compound is selected from aspartic acid,asparagine, glutamic acid and glutamine.
 16. A composition according toclaims 13 or 14 in which the compound is selected from arginine,ornithine and lysine.
 17. A composition according to claims 13 or 14 inwhich the compound is a phosphatidylcholine.
 18. A composition accordingto any one of claims 13 to 17 in which the polyester is capable of over30% crystallinity in the absence of plasticiser.
 19. A compositionaccording to claim 18 in which the polyester m=3 units and m=4 unitshaving respectively a C₁ and C₂ side chain on the carbon atom next tooxygen in the chain are copolymerised together.
 20. A compositionaccording to claim 19 in which the polyester contains 70-90 mol % of m=3units, the balance being m=4 units.
 21. A composition according to anyone of claims 17 to 19 in which the polyester contains less than 2000ppm w/w of nitrogen as N.